For point to point communication, narrow beam or so called "pencil beam" antennas are desirable because they confine their radiated energy into a small area and thereby waste less energy than conventional antennas. In a communications system, as antenna beamwidth is reduced, less power is required to achieve a suitable signal level at the remote receiver. However as antennas are constructed to produce narrower beams, their size and complexity increase. One way that has been developed to reduce antenna beam width is to construct an antenna of moderate beamwidth and then place a radiant energy lens in front of it. A suitably designed lens can focus the antenna beam into almost any desired shape. The antenna itself need only have a beamwidth that adequately illuminates the lens.
Focusing lenses for radio wavelengths have been constructed from dielectric materials, metal strips and plates, and combinations thereof. Another form of lens has been constructed by forming a surface comprising a large number of simple antennas in a planar array having a predetermined shape. The antennas are connected on a one to one relationship with a similar array oppositely faced. The electrical length of all the interconnections can be made the same, or used as a variable parameter, and the shape of the antenna array surface formed into a suitable configuration. Alternatively the array surfaces can be made flat or planar and the length of the interconnection elements varied to give the desired electrical effect. In addition both the surface contour and the variable interconnection line length can be employed simultaneously to give almost any desired lens action. For example a plano to concave or convex surface can be used or concave to convex of reasonable curvature can be achieved. It has been found that such lenses can be designed to enhance either the beamwidth or reduce sidelobe intensity of an antenna. In fact beamwidth reductions along with reduced side lobes have been accomplished. Additionally a lens can be used to either increase or decrease as desired an antenna lobe scanning angle.
Still further it has been found that a single lens can be used to improve the performance of a plurality of separate antennas used to communicate with a plurality of divergent remote locations. This latter aspect is of great importance in satellites. When a satellite is to be used to communicate with a number of widely separated ground stations, a narrow antenna beam cannot be used to cover all locations simultaneously. More desirably one antenna can be used for each ground station and a very narrow beam can be employed for each thereby greatly reducing the power required. In addition such a system admits of separate communications to each ground station using multiple feeds and a common lens aperture with good isolation therebetween.
IN THE TERMS OF A SYNCHRONOUS SATELLITE, THE EARTH SUBTENDS AN ANGLE OF ABOUT 18.degree.. Designing a single lens that will cover such an angle for use with a plurality of separate feed antennas is quite feasible and such a system has the advantage of reduced weight.
Unfortunately the problems of constructing arrays of interconnected antennas by conventional means are formidable. The simple antennas themselves are a problem to make and mount and the interconnecting elements pose a problem in maintaining the required signal delay (or electrical length) along with reliable and easily established connections.